Prepare toppings: In a large bowl, combine onions, jicama, corn and 2 tbsp lime juice. Season with salt and pepper. In a second large bowl, toss cabbage with 4 tsp lime juice and season with salt. In a third medium bowl, mash avocado to a chunky consistency; mix in cilantro and remaining 2 tsp lime juice.
Divide avocado, jicama mixture, cabbage and lobster among tortillas.